A common method for detecting leaks in a pressurized pipeline, such as a pipeline for delivering motor fuel from an underground storage tank to a fuel dispenser at a retail fuel station, is to pressurize the pipeline and to then monitor the pipeline pressure over a period of time. If a leak exists in the pipeline, then the pressure in the pipeline will drop accordingly. The rate of the pressure drop is typically proportional to the size of the leak in the pipeline. For example, a larger leak will result in a faster pressure drop, and a smaller leak will result in a slower pressure drop. Some liquids, such as motor fuels, contained in the pipeline have a high coefficient of thermal expansion that may affect the rate of change of the pressure in the pipeline. In some instances, the thermal expansion of the liquid and/or the air contained in the pipeline may mimic a pipeline leak when no leak exists or may mask a leak when a leak does exist, thus leading to a false conclusion regarding pipeline integrity or tightness.
Precision leak tests of fuel delivery systems are required to conform to performance requirements set forth by federal and state mandates. These precision leak tests, such as testing for a 0.2 gallon per hour (GPH) leak or for a 0.1 GPH leak, are often susceptible to errors induced by the thermal expansion of the fluid in the pipeline and other thermal effects. One method of reaching a reliable 0.1 GPH or 0.2 GPH leak test conclusion involves performing a series of individual leak tests in succession and waiting for the results to stabilize, thereby indicating thermal stability of the product contained in the pipeline. This process can take several hours depending on the pipeline size and thermal conditions. Retail fuel stations ordinarily must shut down the fuel delivery system in order to perform these leak tests. Because of the time required to achieve thermal stability and to complete the leak tests, busy retail fuel stations often have difficulty complying with the leak detection precision required by the government-mandated standards.